Fits, Tolerances and Limits
Overview
Fits, Tolerances and Limits is a fundamental chapter in ITI Fitter Semester 1 under NCVT. It deals with the permissible variation in the size of machined parts so they can assemble correctly. Understanding limits ensures interchangeability of parts in mass production. Tolerance defines the acceptable range of dimensions, while fit describes the relationship between mating parts. This chapter is heavily tested in NCVT theory papers with numerical and conceptual questions.
Typed Notes
Limits and Tolerances — Basic Definitions
A nominal size is the theoretical dimension stated on a drawing. The actual size must fall within two permissible limits: the Upper Deviation (ES for hole, es for shaft) and Lower Deviation (EI for hole, ei for shaft). Tolerance is the algebraic difference between the upper and lower limits. For example, if a shaft is specified as 25 +0.02/−0.01 mm, the upper limit is 25.02 mm, lower limit is 24.99 mm, and tolerance is 0.03 mm. Unilateral tolerance varies in one direction only; bilateral tolerance varies in both directions from nominal size.
Types of Fits
Clearance Fit: The shaft is always smaller than the hole, giving a positive allowance. Used in bearings and sliding parts (e.g., piston in cylinder). Interference Fit: The shaft is always larger than the hole, giving a negative allowance. Assembly requires pressing or heating (e.g., gear on shaft, bush in housing). Transition Fit: The tolerance zones of hole and shaft overlap; the fit may result in either slight clearance or slight interference depending on actual sizes. Used where accurate location is needed (e.g., coupling hubs, locating pins). All three fits are defined by the relative positions of tolerance zones.
Hole Basis and Shaft Basis Systems
In the Hole Basis System, the hole size is kept constant (fundamental deviation = zero, designated 'H') and the shaft size is varied to obtain different fits. This is preferred because holes are harder to machine and measure. In the Shaft Basis System, the shaft size is kept constant and the hole is varied. BIS/ISO uses capital letters (A–ZC) for holes and small letters (a–zc) for shafts to denote fundamental deviations. The combination of fundamental deviation letter and IT grade number gives the tolerance symbol, e.g., H7/f8 (clearance fit), H7/p6 (interference fit), H7/k6 (transition fit).
ISO Tolerance Grades and Allowance
The International Tolerance (IT) grades range from IT01 (finest) to IT16 (coarsest). Finer grades (IT01–IT4) are used for gauges and precision instruments. Medium grades (IT5–IT10) are used for general engineering fits. Coarser grades (IT11–IT16) are used for rough machining and sheet metal work. Allowance is the intentional difference between the maximum material limits of mating parts. Positive allowance = minimum clearance (clearance fit). Negative allowance = maximum interference (interference fit). Allowance ensures proper functioning of the assembly and is always the tightest condition of fit.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Basic Definitions — Size, Limits and Tolerance
- Nominal Size vs Actual Size vs Basic Size
- Upper Limit, Lower Limit and their deviations (ES, EI, es, ei)
- Unilateral vs Bilateral Tolerance
- Tolerance Zone and its graphical representation
A shaft specified as Ø30 −0.00/−0.02 mm has upper limit 30.00 mm, lower limit 29.98 mm, and tolerance = 0.02 mm — this is unilateral tolerance used on a lathe-turned shaft.
02Types of Fits
- Clearance Fit — always positive allowance
- Interference Fit — always negative allowance, press/shrink assembly
- Transition Fit — may be clearance or interference
- Practical applications of each fit type
A 25 mm diameter bronze bush pressed into a cast-iron housing uses Interference Fit H7/p6; the shaft is 0.012–0.042 mm larger than the hole, requiring a hydraulic press for assembly.
03Hole Basis and Shaft Basis Systems
- Hole Basis System — hole kept constant (H), shaft varied
- Shaft Basis System — shaft kept constant (h), hole varied
- Fundamental Deviation letters for holes (A–ZC) and shafts (a–zc)
- Why Hole Basis is preferred in practice
In a lathe headstock, the spindle bearing fit H7/f7 uses Hole Basis System: the 40 mm hole is fixed at H7 tolerance, and the shaft is machined to f7 to give running clearance of 0.025–0.075 mm.
04ISO Tolerance Grades (IT Grades)
- IT01, IT0, IT1–IT16 — range from finest to coarsest
- IT1–IT4 for gauges and precision instruments
- IT5–IT10 for general engineering and fits
- IT11–IT16 for rough machining and press work
A plug gauge for checking a 20 mm hole is manufactured to IT3 grade (tolerance ≈ 0.004 mm), while a rough forged component is accepted at IT14 grade (tolerance ≈ 0.52 mm).
05Allowance and Fit Symbols
- Allowance = tightest condition of mating parts
- Positive allowance (clearance fit) vs Negative allowance (interference fit)
- Reading fit symbols: e.g., Ø25 H7/g6
- Common fit combinations and their use in Fitter trade
Fit symbol Ø50 H8/f7 on a drawing means: 50 mm nominal size, Hole tolerance H8 (0 to +0.039 mm), Shaft tolerance f7 (−0.025 to −0.050 mm), giving minimum clearance (allowance) = 0.025 mm — suitable for a sliding fit in a machine guide.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Tolerance = Difference between Upper Limit (High Limit) and Lower Limit (Low Limit) of a dimension.
- 02Three types of fits: Clearance Fit, Interference Fit, and Transition Fit.
- 03Hole Basis System is preferred over Shaft Basis System in Indian industry practice.
- 04Allowance = Minimum Clearance (positive) or Maximum Interference (negative) between mating parts.
- 05BIS (Bureau of Indian Standards) follows ISO system with 18 grades of tolerance (IT01, IT0, IT1 to IT16).
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
Define tolerance and explain the difference between unilateral and bilateral tolerance with one example each.
What is the difference between Clearance Fit, Interference Fit and Transition Fit? Give one industrial example for each.
Why is the Hole Basis System preferred over the Shaft Basis System in engineering practice?
A shaft has a nominal size of 40 mm with upper deviation +0.025 mm and lower deviation +0.009 mm. Calculate: (a) Upper Limit, (b) Lower Limit, (c) Tolerance.
What are IT grades? Name the IT grade range used for (a) precision gauges, (b) general engineering fits, and (c) rough machining.
Fits, Tolerances and Limits govern the accuracy of machined parts for interchangeable assembly. Tolerance is the permissible size variation; fits (clearance, interference, transition) define how parts mate. The Hole Basis System (H-designation) is standard practice. BIS follows ISO with IT01–IT16 grades. Allowance is the tightest fit condition. Mastering these concepts is essential for NCVT ITI Fitter theory and practical papers.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which of the following fits always has a positive allowance?
Q2.In the Hole Basis System, the fundamental deviation of the hole is designated by the letter:
Q3.A shaft of 30 mm has upper limit 30.02 mm and lower limit 29.99 mm. What is the tolerance on the shaft?
Q4.Which IT grade is used for manufacturing precision gauges?
Mark karo — dashboard par progress dikhega.